The climate varies naturally on all time­scales. Variations can be caused by external forces such
as volcanic eruptions or changes in the sun's energy output. They can also result from the internal
interactions of the climate system's various components – the atmosphere, oceans, biosphere, ice
cover, and land surface. These internal interactions can cause fairly regular fluctuations, such as the El
Nino/Southern Oscillation (ENSO) phenomenon, or apparently random changes in climate.

Natural variability often leads to climate extremes. On time­scales of days, months, and years,
weather and climate variability can produce heat waves, frosts, floods, droughts, avalanches, and severe
storms. Such extremes represent a significant departure from the average state of the climate system,
irrespective of their actual impact on life or the earth's ecology. Record-breaking extremes occur from
time to time in every region of the world.

Growing human vulnerability is transforming more and more extreme events into climatic disasters. A
climate extreme is called a climatic disaster when it has a major adverse impact on human welfare. In some
parts of the world, climatic disasters occur so frequently that they may be considered part of the norm.
Vulnerability to disasters is increasing as growing numbers of people are forced to live in exposed and
marginal areas. Elsewhere, greater vulnerability is being caused by the development of more high­value
property in high­risk zones.

Climate change is expected to increase the frequency and severity of heat waves. More hot weather will
cause more deaths and illnesses among the elderly and urban poor. Together with increased summer drying, it
will lead to greater heat stress for livestock and wildlife, more damage to crops, more forest fires, and
more pressure on water supplies. Other likely impacts are a shift in tourist destinations and a boost in
demand for energy. Meanwhile, fewer cold snaps should reduce cold-related risks to humans and agriculture and
reduce the energy demand for heating while extending the range and activity of some pests and diseases.

More intense rainfall events may lead to greater flooding in some regions. Global warming is expected
to accelerate the hydrological cycle and thus raise the percentage of precipitation that falls in violent
bursts. In addition to floods, this could contribute to more landslides, avalanches, and soil erosion.
Greater flood runoff could decrease the amount of surface water captured for irrigation and other purposes,
although it could help to recharge some floodplain aquifers.

The intensity of tropical cyclones is likely to worsen over some areas. The risks include direct
threats to human life, epidemics and other health risks, damage to infrastructure and buildings, coastal
erosion, and destruction of ecosystems such as coral reefs and mangroves.

Major climate patterns could shift. Although centered in the Southern Pacific, the El Nino/Southern
Oscillation (ENSO) phenomenon affects the weather and climate in much of the tropics. Climate change could
intensify the droughts and floods that are associated with El Niño events in these regions. Similarly,
new patterns could emerge for the Asian summer monsoon, which affects large areas of temperate and tropical
Asia. Likely impacts would include a greater annual variability in the monsoon’s precipitation levels,
leading to more intense floods and droughts.

It is difficult to predict local and regional trends for extreme events. For example, a warming of the
tropical oceans would by itself be expected to increase the frequency, and perhaps the severity, of tropical
cyclones. But other factors, such as changing winds or storm tracks, might offset this effect at the local
level. Another example: because climate models are poor at representing small-scale events, they tend to
disagree on whether or not the intensity of mid-latitude storms will change.

While extreme events are inherently abrupt and random, the risks they pose can be reduced. Improved
preparedness planning is urgently needed in many parts of the world, with or without climate change. Better
information, stronger institutions, and new technologies can minimize human and material losses. For example,
new buildings can be designed and located in ways that minimize damage from floods and tropical cyclones,
while sophisticated irrigation techniques can protect farmers and their crops from droughts.

Climate change also has the potential to cause large-scale singular events. Unlike most extreme
events, singular events would have broad regional or global implications and be essentially irreversible.
Examples of such calamities would include a significant slowing of the ocean’s transport of warm water
to the North Atlantic (which is responsible for Europe’s relatively benign climate), a major shrinking
of the Greenland or West Antarctic ice sheets (which would raise sea levels by three metres each over the
next 1,000 years), and an accelerated warming due to carbon cycle feedbacks in the terrestrial biosphere, the
release of carbon from melting permafrost, or the emission of methane from coastal sediments. Such risks have
not yet been reliably quantified, but fortunately they are expected to be quite low.